The protective effect of selenium against methylmercury toxicity on the nervous system is investigated. Adult male Sprague-Dawley rats will be exposed to methylmercury chloride via intubation at dose level of 2.0 mg/kg body weight (2.0 ppm). One group of mercury-treated animals will also be administered with 0.2 or 0.4 ppm sodium selenite. The blood level of glutathione peroxidase of the animals will be monitored in correlation to the toxic condition of the animals. The animals will be sacrificed in different periods of the experiment according to the development of neurological symptoms of methylmercury poisoning. Before sacrifice, 14C-glycine and 14C-orotic acid will be administered to the animals. The activities of protein synthesis and RNA synthesis in the nerve cells of the cerebellum and dorsal root ganglia in the control, MeHg-treated, MeHg/Se-treated, and Se-treated animals will be compared by means of biochemical method (scintillation analysis). Tissue samples from the cerebellum and dorsal root ganglia of these animals will also be obtained for light and electron microscopic analysis. By means of such biochemical and morphological correlated studies, the bases by which selenium protect against methylmercury toxicity may be elucidated.